The high strength achieved by ceramic materials such as, for example, silicon nitride or silicon carbide allows the employment of these materials for structural applications, for example for heat shields or turbine paddles. By contrast to metallic materials, the ceramics retain their high strength up to temperatures far above 1000.degree. C. This and the high resistance to corrosion open up various applications for the ceramic as a construction material, particularly in the high-temperature range.
Technical concepts that hitherto ran afoul of the lack of suitable materials can be realized by employing ceramic. The goal of these concepts is usually lower exhaust gas emissions and saving fuel. One example of this is achieving higher efficiencies in vehicular and stationary gas turbines by increasing the operating temperature. A further example is a lower-loss and quieter drive in ceramic admission and discharge valves of piston motors by reducing the friction and the oscillating masses.
However, problems can also arise given the employment of ceramic parts, these being caused by disadvantages such as brittleness and scatter of the mechanical properties that are typical of ceramics. Ceramics can exhibit manufacture-conditioned errors such as microcracks, pores, agglomerates, etc. These can lead to a local stress elevation under thermally and mechanically loaded operating conditions that cannot be relieved in the ceramic by plastic deformation, but only by the formation and growth of cracks. When the critical load is exceeded, this leads to the failure of the part.
Since these structural faults typical of ceramics occur in a statistical distribution with respect to their frequency, shape and size, the realizability of ceramic parts can only be enhanced when all parts that contain faults above a specific fault magnitude are eliminated by a proof test before their intended use or before integration. In a proof test, all parts are loaded with a test stress that is greater than the maximum stress occurring during operation The magnitude of the test stress is dependent on the maximum use stress and on the desired service life. The latter is in tun likewise dependent on the initial crack length from which, given a known sub-critical crack growth, the time until a critical crack size is reached can be calculated.
The load conditions that occur during operation should be simulated as exactly as possible in the proof test. Particularly given parts wherein the operating stresses are mainly produced by temperature gradients the stress distribution, however, can usually not be produced by purely mechanical testing without having highly elevated stresses arising at locations that are less loaded during operation. This would result in a great number of breakages during the overload test.
Different applications of ceramic parts can produce different part geometries and different load conditions. There is therefore no standard method for an overload lest. Specific overload tests already exist for individual applications. For example, ceramic balls for hip joint prostheses can be tested by a defined impression of a cone into the receptacle bore for the shaft.
Turbo-supercharger rotors and paddle wheels of vehicle gas turbines can be subjected to a test with excess rpm in the cold condition before being built in. Ceramic grinding wheels are also tested in this way. What is disadvantageous about this method is that thermal stresses and vibrations are not taken into consideration, and that it is limited to rotationally symmetrical parts.
An attempt, for example, is made with the assistance of the thermal shock method to simulate the loads during operation induced by thermal stresses. The parts are thereby heated by gas burners. The stresses produced in this way are calculated from the measured temperature distribution. With this method, however, the failure probability of ceramic parts dazing the operation thereof can be only slightly improved. The suspected reason for this is that the temperature distribution cannot be controlled when heating with the burner. As a result thereof, the stress generated in this way does not coincide with the conditions during the intended operation of the part. It is also possible that not all parts of the part to be tested are subjected to a test stress during the check test that exceeds the usual operating stress.